The networking of control units, sensors and actuators using a communications system has increased greatly in recent years in motor vehicle manufacturing. In this context, of main concern is to achieve synergism effects by the distribution of functions to several control units. In this connection, the term distributed systems is used. Communications among the various users (e.g., control units, sensors and actuators) of the communications system take place increasingly via a bus system. The communications traffic on the bus system, access and reception mechanisms, as well as error handling are governed via a protocol.
In the motor vehicle field, the so-called Controller Area Network (CAN) protocol has been established. This is an event-driven protocol, i.e. protocol activities such as transmitting a message are initiated by events which have their origin outside the communications system. Unique access to the communications system is solved by a priority-based bit arbitration. The presupposition for this is that a unique priority is assigned to each message. The CAN protocol is very flexible, and adding additional users and messages is possible as long as free priorities (message identifiers) are still available.
In the case in which the average utilization of the bus system is relatively low, the probability that a user who wishes to transmit a message can indeed transmit its message at once or within a very short latency period, is very high. Since CAN bus systems are typically laid out in such a way that the average utilization of the bus system is low enough, this means that, as a rule, a very rapid access to the bus system is a given. However, the worst case scenario from the point of view of the communications system, namely that all users permanently wish to transmit, produces an infinitely long latency period, at least for those messages whose priority is relatively low. Therefore, a communications system having a CAN bus system is very well suited for the normal operation (very high probabilities for short latency periods), but less well suited for the worst case scenario (low, finite probabilities for very long latency periods).
Another protocol known from the related art is the so-called time-triggered protocol for Class C (TTP/C), which is a purely time-controlled, and thus a deterministic protocol in which redundancy is predefined in a fixed manner. All communications activities on the bus system are strictly periodic. Protocol activities such as transmitting a message are triggered only by progress in (global) time. Access to the communications system is based on the apportionment of time periods during which a user has an exclusive transmission right. The protocol is comparatively inflexible, and adding new users is only possible when the respective time periods were left free ahead of time. The probability that a user obtains access to the bus system when it wishes to do so is independent of the current utilization of the bus system. The latency period is only a function of the distance in time to the next transmitting time. Since the access request of a user is created outside the influence of the communications system (is asynchronous to it), the latency period between access request and authorization to transmit is equally distributed over the entire time interval between two sending times. This probability distribution is very much broader than in the CAN protocol, i.e. the probability of gaining access to the bus system after a very short time is clearly lower. For this, this distribution is localized, and the probability of very long latency periods is zero. The normal case and the worst case scenarios are the same, and, in contrast to the CAN protocol, an upper limit may be stated for the maximum latency period. With that, the TTP/C protocol is suitable for applications in which the worst case scenario has to be tolerated, even if along with that (slight) restrictions have to be accepted for the normal case. Some fields of application of the time-controlled TTP/C protocol are applications in safety-relevant fields (e.g. steer-by-wire, brake-by-wire or generally X-by-wire in motor vehicles) or applications in which the difference between the normal case and the worst case is not great.
Moreover, a possibility of designing a time-controlled protocol more flexibly is known from the prior art. In this case, the entire protocol operates in a time-controlled manner, but certain time ranges are reserved for an event-driven message transmission. Depending on how access within the event-driven time ranges is regulated, the treatments of the normal case and application-specific individual cases may be improved, without the loss of the fundamental ability to treat the worst case. The fundamental orientation of such a communications system is a time-controlled one, and the event control proceeds within the time control in the reserved time ranges. One bus system that works in this manner is the so-called byte flight bus, or the SI bus.
The present invention relates to both an event-driven and a deterministically controlled bus system. Bus systems are likewise included which represent a combination of an event-driven and a deterministic bus system.
It is an object of the present invention, in a data exchange via a bus system, to enable a rapid access to the bus system in the normal case and ensure a finite latency period for the messages to be transmitted in the worst case.